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Full text of "Text Book Of Mechanical Engineering"

602

Efficiency of Steam.

Commencing at o, Fig. 611, draw the co-ordinates ov, ox, for
pressure and distance respectively. Measure 26-36 ft. at OA, and
2116-8 Ibs. at OB; the rectangle AB then shews external work.
Make OD and D E 12" 36 and 2'i times o B respectively; the area
o F is the internal work during evaporation, and D G shews the
work required to raise the water's temperature from 60° to 212°.
Rectangle AB represents the only useful effect, the rest being
expended on internal changes, and the

external work

^~. .          ,. ,

Efficiency of the steam

- - -

5,799

------- r     .-     =    *'    ^

total work         863,096

= '0646

— -

Let us next examine the case of steam at 160 Ibs. pressure
(above atmosphere), as in triple-expansion engines.

i Ib. of steam at 174*7 Ibs. per sq. in. absolute has a specific
volume of     ..........      2*5        cub. ft.

Load on piston = 174*7 x 144    ..... = 25,156   Ibs.

(1)  External work = 25,156 x 2-5     .    .    .    . = 62,890   ft. Ibs.
Temperature of steam     ...,...«= 370°      F.
Latent heat = (966 - "7(370 - 212)} x 772 = 660,369 ft. Ibs.

(2)  Internal work = (660,369 - 62,890)     .    . = 597, 479 ft. Ibs.

(3)  Heat to raise water's temperature

= (370-60)772 = 239,320 ft. Ibs.
And total work = (i) -H (2) + (3)   .    .    . = 899,689 ft. Ibs.

Efficiency of steam

external work _^  62,890
total work    " 899,189

-07.

Which proves that high pressure steam, weight for weight and
without expansion^ is not more economical than low pressure
steam.

Specific Heats of a Gas.—As with solids and liquids
these are the quantity of heat required to raise the temperature
of i Ib. weight through one degree F. But there are two methods
of raising the temperature, the specific heat being a different
quantity for each case. Assuming the gas enclosed in a cylinder
and covered with a loose piston, we may, while supplying heat,
(i) allow the piston to rise freely, or (2) fix it immovably. In

™*~ are heating at constant pressure, and in (2) at constant